PROCESS AND PLANT FOR SEPARATING OFF C2 OR C2+ HYDROCARBONS

Abstract

Process and plant for separating off C2 or Ca. hydrocarbons from a gas stream comprising light hydrocarbons with or without components boiling lower than methane, in which the gas stream which is at elevated pressure is cooled, partially condensed and separated in a first separation stage into a liquid fraction and a gaseous fraction and the liquid fraction is fractionated by rectification in a second separation stage into a product stream essentially comprising C£ or C2+ hydrocarbons and a residual gas stream predominantly comprising lower-boiling components, the gaseous fraction from the first separation stage which is produced downstream of the partial condensation b~~ng separated by rectification in a third separation stage into a liquid fraction and a gaseous fraction and the liquid fraction being fed as reflux to the second separation stage. According to the invention, the last-mentioned liquid fraction (6') is applied (7') as reflux to a first separation stage (Tl), which acts as a rectifier. , The lower region of the backwash tower (T3) is connected to the upper region of the separation. apparatus for separating off the partially condensed ..- part of the gas stream and the separation apparatus is designed as a stripping tower (Tl) for separating off the partially condensed part of the gas stream. FIGURE 3

Full Text

The invention relates to a process for separating off C2 or C2, hydrocarbons from a gas stream comprising light hydrocarbons with or without components boiling lower than methane, in which the gas stream which is under elevated pressure is cooled, partially condensed and separated in a first separation stage into a liquid fraction and a gaseous fraction and the liquid fraction is fractionated by rectification in a second separation stage into a product stream essentially comprising C2 or C2+ hydrocarbons and a residual gas stream predominantly comprising lower-boiling components, the gaseous fraction from the first separation stage which is produced downstream of the partial condensation being separated by rectification in a third separation stage into a liquid fraction and a gaseous fraction and the liquid fraction being fed as reflux to the second separation stage.
The invention further relates to a plant for carrying out the process according to the invention having at least one heat exchanger for cooling and partially condensing the gas stream, having a separation apparatus for separating off the partially condensed part of the gas stream and having a rectification tower for fractionating the partially condensed part of the gas stream and having a backwash tower.
EP-A 0 185 253 discloses a process of the generic type for producing C2+ or C3 hydrocarbons from gas mixtures which essentially comprise light hydrocarbons with or without hydrogen or nitrogen.
The process according to EP-A 0 185 253 makes possible a highly extensive separation of the C2 and C3+ hydrocarbons, in the backwash tower; see, in

particular, Figure 1 in conjunction with the corresponding description of figures. This effect was surprising in the case of the procedure of EP-A-0 185 253 because condensation of the heavy components still present in the gaseous fraction was achieved by contacting with a lighter fraction. In contrast, to date, hydrocarbons which are heavier than the components to be extracted have been used as washing medium for extracting certain hydrocarbons from a gas.
Processes of this type, such as are described, for example, in the said EP-A-0 185 253, primarily serve for separating off ethane or propane from natural gases or other gases, for example refihery off-gases. Furthermore, these processes are suitable for separating off comparable unsaturated hydrocarbons, such as, for example, ethylene or propylene, if these components are present in the gas stream being fractionated, which can be the case with the abovementioned refinery off-gases.
The process according to EF-A-0 185 253 is therefore also used in what is termed the cold fractionation part of ethylene plants. Ethylene may be produced in principle in two different ways. According to the classic way, a cracked gas is firstly subjected to a C1-/C2+ separation in a front-end demethanizer and the C2+ fraction produced in the course of this is then separated by rectification into a C3+ (residue) fraction and a C2 (product) fraction. In the case of the second way, the cracked gas is first subjected to a C3/C2-separation in a front-end deethanizer. The C2- fraction, which still comprises trace levels of C3- hydrocarbons, is then fed to a C1-/C2 separation by rectification and a C1- (residue) fraction and a C2 (product) fraction are-produced in this separation.
The object of the present invention is enhanced production of the C2 or C2 fraction, that is to specify

a process and a plant for separating off C2 or C2. hydrocarbons from a gas stream comprising light hydrocarbons with or without components boiling lower than methane, in which, firstly, the energy consumption of the C1/C2 separation is decreased and, secondly, the yield of C2 or C2 hydrocarbons is increased. Furthermore, the object underlying the present invention is to enable the process according to the invention to be implemented in rectification towers which consist of less expensive materials.
This is achieved according to the invention by means of the fact that the liquid fraction from the third separation stage is applied as reflux to the first separation stage, which acts as a rectifier.
The plant according to the invention is distinguished by the fact that the lower region of the backwash tower is connected to the upper region of the separation apparatus for separating off the partially condensed part of the gas stream and the separation apparatus is designed as a stripping tower for separating off the partially condensed part of the gas stream.
In contrast to the procedure and plant described in EP-A-0 185 253, the liquid fraction from the backwash tower is fed, not to the rectification tower, but to the first separation tower, which acts as a rectifier. By this means, the liquid fraction from the backwash tower can be stripped against the feed gas in the first separation tower, which takes the place of the separator. This causes the total amount of the light components fed to the rectification tower - that is the C1- components - to be decreased and as a result the separation task in the rectification tower to be simplified.
The process disclosed by EP-A 0 185 253 and the process according to the invention for separating off C2 or C2 hydrocarbons and further developments of the

same are described in more detail below with reference to Figures 1 to 4.
In the drawings:
Figure 1; shows a procedure and plant belonging to the prior art, according to EP-A 0 185 253,
Figure 2: shows a first embodiment of the process
according to the invention, the first
separation tower having no intermediate
reflux.
Figure 3: shows a second embodiment of the process according to the invention, the first separation tower having intermediate reflux.
Figure 4; shows a third embodiment of the process according to the invention, the functions of the first and third separation tower being
implemented in a joint separation tower.
In Figure 1, the feed gas stream introduced via
line 1 and which consists of light hydrocarbons with or without components boiling lower than methane has already been pre-treated in a suitable manner for low-temperature fractionation, that is, in particular, those components have been removed which would cause blockages or impermissible corrosion by freezing out. This gas stream is at an elevated pressure of preferably from 25 to 40 bar and has a temperature between ambient temperature and -S0°c.
The gas stream to be fractionated is cooled in heat exchanger El until the majority of the Ca or C2* hydrocarbons to be separated off and produced' condenses. The partially condensed stream is fed from heat exchanger El via line 2 to a separator D and in this is subjected to a phase separation.

The liquid fraction or the condensate from the separator D is fed via line 3 to an expansion valve b, expanded in this and fed via line 3' to the upper region of rectification tower T2•
In rectification tower T2, the feed liquid fraction from separator D is fractionated into a C? or C21. hydrocarbon product fraction and a residual gas stream comprising lower-boiling components. The C2 or C2 hydrocarbon product fraction is taken off via line 17 from the bottom of rectification tower T2 and fed to expansion valve c, expanded in this and removed from the plant as product stream via line 17', A partial stream of this C2 or C2+ hydrocarbon fraction is evaporated in heat exchanger E3 and fed back via line 18 to the bottom region of rectification tower T2. The residual gas stream already mentioned is taken off from the top of rectification tower T2 via line 13 and fed to heat exchanger El . In this, it is cooled, partially condensed and then fed via line 14 to a further heat exchanger E2. In this it is cooled further and partially condensed, before it is fed via line 15 to the backwash tower T3.
The gaseous overhead fraction taken off via line 4 from separator D is also fed to heat exchanger E2, cooled and partially condensed in this and then fed via line 5, in which an expansion valve a is provided, to backwash tower T3 - The partially condensed higher-boiling components in the latter two streams are taken off from backwash tower T3 via line 6, pumped by pump P to the pressure of rectification tower T2 and fed to this as reflux at the top via line 7.
From backwash tower T3, a sidestream can be taken off via line 16, cooled and partially condensed in heat exchanger E2 and then applied as reflux via' line 16' to backwash tower T3*
At the top of backwash tower T3, a gas stream predominantly comprising methane and lower-boiling

components is taken off via line 8 and warmed in heat exchanger E2. The warmed gas scream is then fed via line 9 to an expansion turbine X, in which the peak refrigeration required in the process is generated. The cooled gas stream is fed via line 10 again to heat exchanger E2 and warmed in this. Then it is fed via line 11 to heat exchanger El, warmed again in this against process streams which are to be cooled and discharged via line 12 from the process or plant, for example as a fuel gas stream.
Alternatively to the single- or multistage expansion shown in Figures 1 to 4 by means of one or more turbines, the gas stream taken off via line 8 from backwash tower T3 could also be subjected to a Joule-Thomson expansion in an expansion valve which is not shown in the figures.
Furthermore, alternatively, or additionally, to the expansion using a turbine or the Joule-Thomson effect, external provision of refrigeration can be provided.
The refrigeration required in heat exchanger El for cooling and partial condensation of the feed gas stream and of the residual gas stream from the rectification tower is provided by up to three external refrigerant circuits; in Figures 1 to 3 and 4, for the sake of clarity, only two refrigerant circuits 19 and 20, or three refrigerant circuits, 19, 20 and 29, are shown in each case.
In addition, for the sake of clarity, in Figure 1, only one separator D is shown, However, in reality, two or three - rarely more - separators are connected in series, the bottom fractions of the individual separators being fed to rectification tower T2, while the overhead fractions - apart from the -overhead fraction of the third or last separator - are each fed to the subsequent separator after cooling and partial condensation have been performed.

Figure 2 shows, as mentioned above, a first embodiment of the process according to the invention, the first separation apparatus Tl acting as a rectifier, which is designed as a stripping tower, having no intermediate reflux.
For the sake of simplicity, only the differences between the procedure included in the prior art - as shown in Figure 1 and explained on the basis of this - and the process according to the invention, as depicted in Figures 2 to 4 - are explained below.
The gas stream to be fractionated, which is cooled and partially condensed in heat exchanger El, is then fed via line 2 to a first separation apparatus Tl which acts as a rectifier and is designed as a stripping tower.
In accordance with an advantageous development of the process according to the invention, the cooled and partially condensed gas stream which is at elevated pressure has, prior to its feed into stripping tower Tl, a temperature between -100 and -40°C, preferably between -90 and -55°C.
Whereas, according to the procedure of Figure 1, the liquid fraction produced in the bottom of backwash tower T3 is fed directly to rectification tower T2, this liquid fraction is now taken off via line 6', pumped by pump P' to the pressure of stripping tower Tl and fed to the top regipn of this via line 7'. This procedure has the following advantages: owing to the fact that the liquid bottom fraction of backwash tower T3 is stripped in stripping tower Tl against the feed gas to be fractionated, the total amount of the light components fed to rectification tower T2, that is the amount of methane and components boiling lower than methane, is decreased. By means of^ this process procedure, the separation task in rectification tower T2 is made easier.

In addition, in stripping tower Tl, an advantageous heat exchange takes place, since the liquid fraction from the bottom of backwash tower T3, which was previously fed to a comparatively high and thus unfavourable temperature level at the top of rectification tower T2 - the temperature difference between the top temperature of rectification tower T2 and the temperature of the feed liquid fraction is 36°C -, is fed, in accordance with the process according to the invention to a far more expedient temperature level of stripping tower Tl - the temperature difference between the top temperature of stripping tower Tl and the temperature of the feed liquid fraction is now merely 11°C. The procedure according to the invention thus enables enhanced utilization of the peak refrigeration generated by expansion turbine X. The energy consumption of the C1/C2 separation can thus be markedly reduced.
Figure 3 shows a second embodiment of the process according to the invention, in which stripping tower Tl has intermediate reflux. For this purpose, a sidestream is taken off via line 21 from the lower region of stripping tower Tl, cooled and partially condensed in heat exchanger El and then applied via line 22 to stripping tower Tl as intermediate reflux. By means of this embodiment of the process according to the invention, the separation work in stripping tower Tl is improved.

A further embodiment of the process according to the invention is shown in Figure 4. In this design of the process according to the invention, the functions of the first and third separation tower Tl and T3 - as shown in Figures 2 and 3 - are implemented in a joint separation tower Tl/3. Thus lines 6' and 7'. connecting separation towers Tl and T3, and pump P' disposed in them can be dispensed with. However,

provision of a pump P' in lines 3 and 3' connecting separation towers Tl/3 and T2 is now required.
In principle, separation tower Tl/3 and the rectification taking place in it, shown in Figure 4, can be "fractionated" not only into two separate separation towers Tl and T3, as shown in Figures 2 and 3, but, depending on the number of intermediate cooling take-offs 21, 6' and 16, into a plurality of separate separation towers.
Figure 4 shows, in contrast to Figures 1 to 3, only one heat exchanger El/2. Here also, in principle, a multiplicity of separate heat exchangers can be provided.
The gas stream predominantly comprising methane and lower-boiling components taken off at the top of separation tower Tl/3 via line 8 is, after it is warmed in heat exchanger El/2, fed via line 9 to a first expansion turbine X and, after warming again in heat exchanger El/2, fed via line 9' to a second expansion turbine X', the required process peak refrigeration being generated in these expansion turbines. The cooled gas stream is then fed via line 10' again to heat exchanger El/2, warmed in this and leaves the plant via line 12.
Since, in the process according to the invention, the low-temperature feed of the liquid bottom fraction from backwash tower T3 to rectification tower T2 is dispensed with, rectification tower T2 can be fabricated from less costly materials - low-temperature steel instead of high-alloy steel.
The appropriate compositions, pressures, temperatures etc. according to Figure 3 for a concrete illustrative example are given in the table below.
While the top temperature of rectification' tower T2 in the process shown in Figure 1 is -58°C, in the procedure according to the invention in accordance with Figure 3 it is -34°C. This makes the use of less

costly materials for rectification tower T2 possible, since above a temperature of -40C, low-temperature steel can be used instead of high-alloy steel.

1. Process for separating off C2 or c2-hydrocarbons from a gas stream comprising light hydrocarbons with or without components boiling lower than methane, in which the gas stream which is at elevated pressure is cooled, partially condensed and separated in a first separation stage into a liquid fraction and a gaseous fraction and the liquid fraction is fractionated by rectification in a second separation stage into a product stream essentially comprising C2 or C2+ hydrocarbons and a residual gas stream predominantly comprising lower-boiling components, the gaseous fraction from the first separation stage which is produced downstream of the partial condensation being separated by rectification in a third separation stage into a liquid fraction and a gaseous fraction and the liquid fraction being fed as reflux to the second separation stage, characterized in that, the last-mentioned liquid fraction (6') is applied (7') as reflux to the first separation stage (Tl), which acts as a rectifier.
2. Process according to Claim 1, characterized in that the gaseous fraction (4) from the first separation stage (Tl) produced downstream of the partial condensation is cooled and partially condensed (E2) prior to its feed into the third, separation stage (T3).
3. Process according to Claim 1 or 2, characterized in that the residual gas stream (13) from the second separation stage (T2) is cooled, partially condensed (El) and fed to the third separation stage (T3)
4. Process according to Claim 2 or 3, characterized in that the gaseous fraction (4) produced downstream of the partial condensation and/or the residual gas stream (13), prior to the feed into the third separation stage (T3) is/are cooled in indirect heat exchange (E2) with external refrigeration and/or

process streams to be warmed, in particular in indirect heat exchange with the residual gas stream (8) of the third separation stage (T3) which was expanded by a valve or at least one turbine (X, X').
5. Process according to one of the preceding
claims, characterized in that at least one partial
stream (21) is taken off from the first separation
stage (Tl) which acts as a rectifier, cooled, partially
condensed and fed as intermediate reflux (22) to the
first separation stage (Tl).
6. Process according to one of the preceding
claims, characterized in that the temperature of the
cooled and partially condensed gas stream (2) which is
at elevated pressure is, prior to the feed into the
first separation stage (Tl), which acts as a rectifier,
between -100 and -40°C, preferably between -90 and
-55°C.
7. Plant for carrying out the process according to
one of the preceding claims, having at least one heat
exchanger for cooling and partially condensing the gas
stream, having a separation apparatus for separating
off the partially condensed part of the gas stream, and
having a rectification tower for fractionating the
partially condensed part of the gas stream, and having
a backwash tower, characterized in that the lower
region of the backwash tower (T3) is connected to the
upper region of the separation apparatus for separating
off the partially condensed part, of the gas stream, and
V V ,
the separation apparatus is designed as a stripping tower (Tl) for separating off the partially condensed part of the gas stream.

9. Process and plant for separating off C2 or C2+ hydrocarbons from a gas stream substantially as described hereinabove and illustrated with reference to the accompanying drawings.